How do I tell my friend that she is toxic and sheared ruining another person’s love life

I (16F) have these two friends, Jeremy (17M) and Sarah (16F), and they have been in a “situationship” (situationship is a very loosely used) for about a year and a half. Jeremy is HEAD OVER HEELS for Sarah but she has just been playing him on and off. I know both point of view because Im close friends with both of them, and they have two VERY different looks on things. Jeremy thinks she is the girl of his highschool life, while Sarah doesn’t give a crap about him and is just using him for attention. I’ve been trying to get him to drop her, but he doesn’t listen. She has been trying to get either other guys and stuff, which is no biggie because they aren’t together, but is shitty because she is just leading him on. I’ve ran out of options and don’t know how to confront her. How do I deal with this situation without loosing my friends or ruining their friendship?